### Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle 523 du LPTMC - Tour 12-13

**Pressure and forces in active matter**

**Alexandre Solon (LPTMC)**

Active matter, composed of self-propelling entities, is found across scales in nature, from cellular tissues to animal groups. Such systems, as well as engineered active materials, exhibit many types of collective behaviors and unusual mechanical properties. In this talk, I will focus on different aspects of the interactions between active fluids and boundaries or passive objects, and show that they lead to intriguing effects, specific to active systems. In particular, I will discuss the absence of equation of state for the pressure of active fluids, the instability of a filament in an active bath, long-range interactions mediated by an active fluid and the localization of active particles in a random potential.

**Emergent phenomena from correlation and competition: a case of figure from high temperature superconductivity**

**Marcello Civelli (LPS Orsay)**

Emergent phenomena are common in correlated systems. For instance, in correlated quantum materials novel phases appear displaying remarkable properties, like e.g. colossal magneto-resistance, exotic charge and magnetic order, quantum criticality, unconventional superconductivity….Tuning the system from one phase to another via the application of an external parameter, like temperature, pressure, doping, or by manipulating intrinsic parameters via, for example, light-matter interaction, offer the possibility to exploit a large variety of different functionalities on the same device and opens the route to the development of novel technologies.

Two main concepts that frame the emergence of novel phenomena from correlation have been at the front stage. On one side the competition between quantum phases, famously advocated for example in the case of quantum critical phase transitions in heavy fermion compounds. On the other side, the idea that “more is different” (P.W. Anderson), i.e. that our understanding of these new quantum phenomena require a new quantum state of matter, sharply different from the ones that we have known so far.

We shall show how these two concepts merge in the most known exotic state of matter rising from correlation, the high-temperature superconductivity in cuprates. The scenario that we propose is obtained by resolving the Hubbard Model, which is the simplest model capturing the salient features of cuprates. In particular we shall show that an unprecedented form of superconducting pairing arises from correlation, requiring to go beyond the main concepts of unconventional superconductivity developed so far.

**Unbinding transition of probes in single-file systems**

**Vincent Démery (Gulliver-ESPCI)**

Single-file transport, arising in quasi-one-dimensional geometries where particles cannot pass each other, is characterized by the anomalous dynamics of a probe, notably its response to an external force. I will present a simple hydrodynamic framework that allows to compute this response, and also the response of several probes to arbitrary external forces, where an unbinding transition can occur.

Séminaire commun avec le LPTHE. Horaire et salle exceptionnels: **bibilothèque du LPTHE (tours 13-14, 4ème étage) vendredi à 11h**

**Dynamics in quantum antiferromagnets**

**Maxime Dupont (U.C. Berkeley)**

Theoretically challenging, the understanding of the dynamical response in quantum magnets is of great interest, in particular for both inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments. In this talk, I will address this question for quasi-one-dimensional quantum magnets, e.g., weakly coupled spin chains for which many compounds are available in nature. In this class of systems, the dimensional crossover between a three-dimensional ordered regime at low temperature towards one-dimensional physics at higher temperature is a nontrivial issue, notably difficult concerning dynamical properties. I will present a comprehensive theoretical study based on both analytical calculations and numerical simulations which allows us to describe the full temperature crossover for the NMR relaxation rate 1/T1, from one-dimensional Tomonaga-Luttinger liquid physics to the three-dimensional ordered regime, as a function of interchain couplings.

**Compact packings of spheres**

**Thomas Fernique (LIPN, CNRS et U. Paris 13)**

It is well known that the best way to pack oranges in a (very large) box is to place them on a face-centered cubic lattice (also known as checkerboard), although this has been formally demonstrated only in 1998 (with difficulty). This talk focuses on what happens when the dimension or number of different spheres change. In particular, so-called compact packings (the term will be defined properly) seem good candidates to maximize density. In this tallk, we propose : a) a non-technical survey of the known mathematical results ; b) an overview of the underlying computer problems (interval arithmetic, resolution of systems of polynomial equations, combinatorial exploration) ; (c) a discussion of possible applications in chemistry, including self-assembly of supercrystals ; d) a discussion of numerical simulations that may be of interest in this context.